Late Cretaceous protolith age and provenance of the Pelona and Orocopia schists, Southern California; implications for evolution of the Cordilleran margin

The Pelona and Orocopia Schists are southern members of a eugeoclinal terrane that structurally underlies a large part of southwestern North America. Ion-microprobe U-Pb ages of >100 detrital zircons from three widely spaced samples of these two units indicate that deposition occurred after 70-80...

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Bibliographic Details
Published in:Geology (Boulder) 2000-03, Vol.28 (3), p.219-222
Main Authors: Jacobson, Carl E, Barth, Andrew P, Grove, Marty
Format: Article
Language:English
Subjects:
absolute age
Baja British Columbia
California
Cretaceous
dates
deformation
detritus
faults
Geochronology
Geological time
igneous and metamorphic rocks
Mesozoic
metamorphic rocks
metasedimentary rocks
Mojave Desert
nesosilicates
North America
North American Cordillera
Orocopia Schist
orthosilicates
Pelona Schist
Petrology
Precambrian
Precious stones
protoliths
provenance
schists
silicates
Southern California
terranes
Transverse Ranges
U/Pb
underthrust faults
United States
Upper Cretaceous
zircon
zircon group
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Summary:The Pelona and Orocopia Schists are southern members of a eugeoclinal terrane that structurally underlies a large part of southwestern North America. Ion-microprobe U-Pb ages of >100 detrital zircons from three widely spaced samples of these two units indicate that deposition occurred after 70-80 Ma. Moreover, the distribution of zircon ages, including a significant peak centered ca. 1.7 Ga, implies a major contribution of detritus from the Mojave Desert and Transverse Ranges of southern California. Recrystallization of the schists at depths of 20-35 km occurred within 10-15 m.y. of deposition, which requires underthrusting at minimum rates of ∼4-12 mm/yr for reasonable thrust dips. Considering that similar processes formed the more northern Rand Schist at somewhat earlier times, our results indicate a southward progression in timing of deformation for the schists' protoliths. This result is at odds with northward migration of deformation implied by the Baja British Columbia hypothesis. The age and provenance of the schist's protoliths are consistent with models that derive the schists from either the Franciscan Complex or Great Valley Group.
ISSN:0091-7613
1943-2682
DOI:10.1130/0091-7613(2000)28<219:LCPAAP>2.0.CO;2